1. Field of the Invention
The invention relates to a method and an apparatus for the alignment of paper stacks in a printing press to provide a minimum amount of strain to the control and operation of the sheet feeding function of the printing press.
2. Background Information
In the printing industry, there is often a need for printing a large number of items. In order to do so, it can be beneficial if the supply of paper to the printing press is essentially continuous, thereby providing essentially little down time for the operation of the press. For the printing press to operate successfully during the transfer of the paper supply from a first stack to a second, it can be beneficial to the operation if the last sheet of the stack being used is substantially aligned with the first sheet of the next stack to be used. Thus, there would essentially be very little, to preferably no lateral offset between the last sheet of the first stack and the first sheet of the next stack to be used. Any amount of lateral offset could possibly strain the regulation and control of the printing machine, and could even result in a paper jam, especially if the lateral offset is severe.
In order to compensate for any problems which might occur in the transfer of sheet delivery from one stack to another, it is preferable that the stacks be substantially aligned so that lateral position of the last sheet of the stack being used is essentially aligned with the lateral position of the first sheet in the next stack, thereby providing a relatively small amount of lateral offset between the paper stacks so that little or no strain results in the regulation and control of the machine.
A known method for aligning paper stacks on a printing press includes exposing the paper stacks to optical light by means of a transmitter, and then evaluating the angular correlation of the rays of light reflected from the stacks. The reflected light rays can be detected by a sensor with a position-sensitive, photo-receiving arrangement.
In the alignment of paper stacks, as well as in several other cases of distance or positional measurement of objects, it is not so much the absolute distance that is of interest, but rather the relative position, for example, of two objects relative to each other, i.e. the position of one paper stack relative to the position of another paper stack which will be used to replace the first stack.
This is typically the case in a variety of different industries, for example, with positioning and follow-up tasks in the steel industry, with packaging machines in the paper and printing industry, and in the building industry, etc. For example, in the printing industry, as discussed above, during the operation of a printing machine, a new paper stack often needs to be aligned with, and then united with an auxiliary paper stack to replenish the paper supply. To achieve this, the relative position of both stacks must be measured to position the new paper stack in alignment with the auxiliary stack that is being used.
For such tasks, known optical triangulation sensors and control elements can be used, whereby the relative position of any two objects can be determined from the difference values between the output signals received by two such sensors. With such a procedure, there is a definite disadvantage that measuring errors which independently occur in both sensors can multiply considerably, so that in some circumstances the use of such a measuring method with two sensors may become essentially impossible. A precise uniformity of the characteristic curves and a low degree of drifting or wear of such devices must especially be demanded. Such precision makes these devices very expensive, and in many cases they can reach their physical limits.